1. Field of the Invention
This invention relates to improvements in electrochemical cells or batteries and more particularly to zinc halogen cells or to batteries of cells preferably using chlorine as the halogen and excluding fluorine but it may be iodine or bromine preferably in gaseous form. Such a cell or battery may be as described in United Kingdom Letters Patent Specification No. 1,258,502, U.S. application Ser. No. 804,215, now U.S. Pat. No. 4,105,829.
2. Description of Prior Art
British Pat. No. 1,258,502, U.S. application Ser. No. 804,215 describes a cell or a battery of cells, the cell comprising a casing, a porous halogen storing electrode forming the cathode, a zinc bearing electrode forming the anode, the electrode being immersed in a liquid zinc halide electrolyte solution, a halogen gas inlet to the casing, a gas flow-path in the casing from the gas inlet to the interstices of the halogen storing electrode, and positive and negative electric conductors leading from the electrodes to terminals accessible to the outside of the casing. The cell is in a halogen gas circuit including a halogen gas cylinder, a pressure regulated valve to control the flow of gas from the cylinder to the casing interior, an ultra violet lamp for the removal of undesirable gases such as hydrogen from the circuit, and a halogen gas liquifier connected to the cylinder, the latter being in the circuit between the valve and the liquifier. An aqueous return may be provided from the liquifier to the cell to return water, which normally has other constituents, into the cell or battery electrolyte. The porous carbon electrode acting as a cathode is made up of a number of electrode structures each comprising a substrate of anodizable metal selected from the metals of Group IV(A) and Group V(A) of the Periodic Table according to Mendeleef having permanently associated therewith at least one coherent stratum of substantially porous carbon. The preferred anodizable metal is titanium. The substrate is preferably in the form of an open mesh such as expanded metal, an apertured sheet of the metal or a porous sheet of the metal having a pore size for example of 3-25 thousandths of an inch diameter, and perferably having a pore size larger than the size of the carbon particle in the crumb, which is the preferred form of the carbon particles and binder. When titanium is used, it is preferable to use titanium of commercial purity having mechanical hardness of I.M.I cp 115 or 130. Zinc bearing electrode forming the anode may be formed by a titanium mesh as for the cathode which is coated or otherwise treated with zinc. The crumb is pressed onto the substrate at a pressure of at least 2 tons psi.
The cathode may be made up of the electrode structure, the spaces in each cathode between adjacent cathode structures being used to supply the chlorine gas to the surfaces of the porous carbon of the structure.
In such a cell or battery during discharge the halogen e.g. chlorine in gaseous form, is supplied to the cathode from the external storage system and on recharging, the halogen gas is liberated on the surfaces of the cathode, rises to the surface of the electrolyte, escapes or is removed from the cell casing, compressed and stored for reuse in a subsequent cycle. The gas may be treated to remove hydrogen and other impurities before it passes to the storage capacity. In operation the halogen gas if frequently wet which necessitates the use of materials inert to the corrosive gas for making the ancillary equipment such as ducts and storage capacities, e.g. high pressure cylinders used in the aforesaid cells or batteries.
It has been found in operating such cells under an internal pressure equal to atmospheric pressure the cell is polarisation limited in output due to the diffusion rate limitation of the halogen e.g. chlorine into the electrolyte solution.
The main object of the present invention is to provide a cell or battery of the zinc halogen type in which the power to weight ratio is increased.
It is known that the solubility of chlorine gas in zinc chlorine solution is a function of the temperature and pressure in the space within which the halogen and zinc chloride are contained, so that an increase in pressure within the space permits a greater concentration of the chlorine in solution where it becomes the active cathode. It is also known that solvents exist for the halogens which exhibit this function at ambient temperatures and pressures, suitable solvents for chloride for example being carbon tetrachloride and sulphuryl chloride and such solvents can be in liquid or solid form. It is also known that chlorine gas at normal atmospheric temperatures e.g. 2.degree.-28.degree. C. is liquified at a pressure of about 75 lbs/sq.in but at any pressure above 148.degree. C. chlorine is unliquifyable.